**5. Condensation curves and methane numbers**

The conditioning with air and / or liquid gas to adjust the technical combustion characteristics may influence both the methane number and the condensation of higher hydrocarbons in the combustion gas mixture.

The methane number - equivalent to the octane number of petrol - is a statement of the antiknock properties of fuel combustion in a engine, where the term anti-knock refers to the tendency to uncontrolled and undesirable self-ignition. Methane has by definition a methane number of 100, hydrogen a methane number of 0. A methane number of 80 for example means that the gas mixture associated with this methane number has the same antiknock properties as a mixture of 80% vol. methane and 20 vol. -% hydrogen. Some inert mixture components such as CO2 increase the methane number, higher hydrocarbons, reduce it. The calculated methane numbers (Gascalc, E.on Ruhrgas) of L gases are generally greater than those of H gases (nitrogen not factored out). As a lower limit for the smooth

Figures 11 and 12 show the calorific values and the Wobbe index for an air and LPG admixture of 0 to 20 vol -% with an initial methane content of 98 vol -% and 99,5 vol -%.

Fig. 12. Possible highly calorific L gas mixtures by admixing air and LPG to an initial

The red area represents the required calorific value range from 9.97 to 10.4 kWh / m³. The green dots show the possible, compliant mixtures that lie within all the conditions to be

The conditioning with air and / or liquid gas to adjust the technical combustion characteristics may influence both the methane number and the condensation of higher

The methane number - equivalent to the octane number of petrol - is a statement of the antiknock properties of fuel combustion in a engine, where the term anti-knock refers to the tendency to uncontrolled and undesirable self-ignition. Methane has by definition a methane number of 100, hydrogen a methane number of 0. A methane number of 80 for example means that the gas mixture associated with this methane number has the same antiknock properties as a mixture of 80% vol. methane and 20 vol. -% hydrogen. Some inert mixture components such as CO2 increase the methane number, higher hydrocarbons, reduce it. The calculated methane numbers (Gascalc, E.on Ruhrgas) of L gases are generally greater than those of H gases (nitrogen not factored out). As a lower limit for the smooth

concentration of 99,5 Vol. -% methane

**5. Condensation curves and methane numbers** 

hydrocarbons in the combustion gas mixture.

fulfilled.

operation of modern engines, a methane number of MZ > 70 is considered necessary (DIN 51624, 2008).

For multi-component mixtures such as natural gases, the condensation and boiling curves do not lie together, but span a conditional area, where different gas-liquid compositions are possible. Between the critical pressure and the cricondenbar point with increasing temperature, and between the critical temperature and the criconden therm point with falling pressure, condensate (retrograde condensation) can form when the throttle curve touches the dew line, intersects or the final state lies in the two-phase region (Höner zu Siederdissen & Wundram, 1986).

An admixture of propane / butane to natural gas and processed biogas generally manifests itself in a shift of the dew curve to higher temperatures. According to (Oellrich et al., 1996), in the case of Russian H gas, condensation is only to be expected at temperatures of -35 ° C, while it will occur with Dutch L gas already at -5 ° C. If liquid gas/air is admixed within the limits described in DVGW worksheet G 260, the criconden therm point moves toward +15 ° C or +45 ° C, but at higher pressures. For mixtures of natural gas and processed conditioned biogas, this is to be expected to a lesser degree, since the concentrations of propane / butane are correspondingly smaller.

It should be noted that the calculation of the condensation curves of natural gases requires an analysis that takes into account the higher hydrocarbons, since even small amounts in the ppm range result in a significant shift. Furthermore, the process of condensation is not in itself critical, but the quantity of condensate is the decisive criterion. For large flow rates, a seemingly low volume of condensate can therefore lead to problems (Oellrich et al., 1996).

The following Table 10 and Figure 13 show cases of condensation in conditioning by the addition of LPG, in order to meet the North Sea I specification. The lowest and the highest admixtures were selected for the diagrams. It should be noted that at the highest level of admixing, the restrictions imposed by G 486 were not observed.


Table 10. Cases of condensation for mixtures of the North See I H gas specification

Conditioning of Biogas for Injection into the Natural Gas Grid 389

Restriction of the lower calorific value range by high processing levels

For the practical implementation of the listed quantities of the LPG admixtures, limits as given in Table 12 are to be observed, in accordance with the requirements presented on the need for the use and applicability of SGERG88 and AGA8 procedures and the resulting maximum admixture quantities according to Table 12. Due to these limits, defined in DVGW G 486-B2, it will not be possible in every case to reach the upper calorific value range at higher pressures. In addition, the availability of appropriate measuring technology for higher liquid gas fractions is limited. At the very high degrees of methane processing, there are limitations on attaining the lower calorific value range, since when processing to 99.5

> **Limit according to G 486 supplementary**

**Limit according to G 486 supplementary** 

**sheet 2 Appendix B p> 100 bar** 

**sheet 2 Appendix B p> 100 bar** 

Table 13 shows the maximum possible , compliant LPG admixture, a propane / butane mixture of 95 / 5 Mass .-%. As a result, it is clear that processing to a maximum methane content of 99.5% vol. with the maximum permissible LPG admixtures, a calorific value of maximum 11,361 kWh/m³ (NTP) is possible at pressures above 100 bar, and a maximum of

LPG mixing rates to achieve the target calorific value + / - 2%

North Sea I H gas

11,956 - 12,444 kWh/m³

Table 11. Air and LPG additions of the investigated H gas properties

Vol -% methane, an H gas with a calorific value of 11.009 kWh / m³ results.

Propane xC3H8 mol.-% 3,5 6.

Butane xC4H10 mol.-% 1,5 1,5

Table 12. Limits according to DVGW G 486 supplementary sheet 2 appendix B

HS,n =

LPG

**Descriptor Unit** 

12,075 kWh/m³ (NTP) at pressures below 100 bar.

in Vol.-%

Methane

processing in vol -%

concentration after

94,0 9,4 - 12,6 96,0 8,1 - 11,3 98,0 6,8 - 9,9 99,5 5,8 - 8,9

**Kondensationslinien (SRK-Gleichung)**

**Temperatur T in °C**

Fig. 13. Condensation curves for mixtures of the North See I H gas specification

In summary, it can be said that the criconden therm points of the H gases in Germany lie below temperatures of -20 ° C. An exception to this are the higher caloric mixtures of the North Sea quality - here, 0 ° C is also possible. However, the mixtures were ignored in the calculation of the limits in G 486 and DIN 51 624, so that it applies mainly to the higher LPG quantities added. Generally, this means that process procedures, in which pressure and temperature lie in the two-phase region, should be avoided.

### **6. Conclusion**

This section shows a summary of all the mixing rates of LPG and / or air to attain the base gas properties under consideration.

#### **6.1 Conditioning: Target H gas**

The table 11 shows the determined rates of admixture for LPG to attain the appropriate target calorific value range. With the LPG quantities shown, the respective initial concentrations of methane, the entire calorific value range of the respective base gas quality is covered, with some restrictions.

**Kondensationslinien (SRK-Gleichung)**


**Temperatur T in °C**

In summary, it can be said that the criconden therm points of the H gases in Germany lie below temperatures of -20 ° C. An exception to this are the higher caloric mixtures of the North Sea quality - here, 0 ° C is also possible. However, the mixtures were ignored in the calculation of the limits in G 486 and DIN 51 624, so that it applies mainly to the higher LPG quantities added. Generally, this means that process procedures, in which pressure and

This section shows a summary of all the mixing rates of LPG and / or air to attain the base

The table 11 shows the determined rates of admixture for LPG to attain the appropriate target calorific value range. With the LPG quantities shown, the respective initial concentrations of methane, the entire calorific value range of the respective base gas quality

Fig. 13. Condensation curves for mixtures of the North See I H gas specification

temperature lie in the two-phase region, should be avoided.

0

**6. Conclusion** 

gas properties under consideration.

**6.1 Conditioning: Target H gas** 

is covered, with some restrictions.

10

20

30

40

50

**Druck p in bar**

 **Methan LPG Brennwert 94,00 9,40 11,96 94,00 12,60 12,43 96,00 8,10 11,97 96,00 11,30 12,44 98,00 6,80 11,97 98,00 9,90 12,44 99,50 5,80 11,97 99,50 8,90 12,44 Nordsee I 11,76**

60

70

80

90

100



For the practical implementation of the listed quantities of the LPG admixtures, limits as given in Table 12 are to be observed, in accordance with the requirements presented on the need for the use and applicability of SGERG88 and AGA8 procedures and the resulting maximum admixture quantities according to Table 12. Due to these limits, defined in DVGW G 486-B2, it will not be possible in every case to reach the upper calorific value range at higher pressures. In addition, the availability of appropriate measuring technology for higher liquid gas fractions is limited. At the very high degrees of methane processing, there are limitations on attaining the lower calorific value range, since when processing to 99.5 Vol -% methane, an H gas with a calorific value of 11.009 kWh / m³ results.


Table 12. Limits according to DVGW G 486 supplementary sheet 2 appendix B

Table 13 shows the maximum possible , compliant LPG admixture, a propane / butane mixture of 95 / 5 Mass .-%. As a result, it is clear that processing to a maximum methane content of 99.5% vol. with the maximum permissible LPG admixtures, a calorific value of maximum 11,361 kWh/m³ (NTP) is possible at pressures above 100 bar, and a maximum of 12,075 kWh/m³ (NTP) at pressures below 100 bar.

be considered.

**6.2 Conditioning: Target L gas** 

with a combination of air and LPG.

this limit is never reached.

**LPG-Zugabe [Vol.-%]**

Conditioning of Biogas for Injection into the Natural Gas Grid 391

**LPG-Mengen zur Erreichung des Zielbrennwertes**

Russland H-Gas Nordsee I H-Gas Nordsee II H-Gas Verbund H-Gas

> **LPG-Zumischgrenze bei < 100 bar**

> **LPG-Zumischgrenze bei > 100 bar**

94 96 98 99,5 **Methankonzentration [Vol.-%]**

In order to achieve higher calorific values, alternative conditioning measures can be employed. For example, ready-made mixtures with customized propane / butane ratios can be used for conditioning. This can increase the calorific value, but technical and physical effects, such as condensation behaviour, methane number and k-number deviations need to

Two different L gas target properties have been described. Because of their basic constitutions, one bio-methane mixture is conditioned with air and the other is conditioned

Table 14 shows a summary of the admixtures with which a target calorific value-oriented

In the case of simple air addition, particular attention should be paid to compliance with the maximum O2 volume fraction. This should not exceed 3 % vol. in dry networks according to DVGW worksheet G 260. This quantity is reached when adding pure air to the processed biogas, at an admixture of 15 vol -% of air. In the low caloric L gases (e.g. Weser Ems gas),

Furthermore, a minimum air addition may also be necessary, in order to achieve the

Fig. 14. LPG quantities necessary to achieve the target calorific value

mixture for the low calorific base gas property can be achieved.

required Wobbe Index according to DVGW worksheet G 260.


Table 13. Gas properties with maximum compliant LPG additions

Figure 14 shows the admixture required to achieve the corresponding H gas properties. The limits on the maximum concentration of propane are also shown according to DVGW regulations G 486 supplementary sheet 2 a ppendix B. Admixtures to achieve the properties of North Sea I / North Sea II H gas are, based upon all levels of methane processing, above the limits. A compliant mixture is not possible in this case, or needs to be tested on an individual basis.

Butane content in

admixture

95,000 3,497 0,141 11,151 13,969 0,637

96,000 3,497 0,141 11,257 14,210 0,628

97,000 3,497 0,141 11,364 14,454 0,618

98,000 3,497 0,141 11,471 14,704 0,609

99,000 3,497 0,141 11,577 14,959 0,599

99,500 3,497 0,141 11,631 15,088 0,594

95,000 5,978 0,241 11,605 14,265 0,662

96,000 5,979 0,241 11,709 14,495 0,653

97,000 5,979 0,241 11,813 14,729 0,643

98,000 5,979 0,241 11,917 14,967 0,634

99,000 5,979 0,241 12,021 15,209 0,625

99,500 5,980 0,241 12,073 15,332 0,620

Figure 14 shows the admixture required to achieve the corresponding H gas properties. The limits on the maximum concentration of propane are also shown according to DVGW regulations G 486 supplementary sheet 2 a ppendix B. Admixtures to achieve the properties of North Sea I / North Sea II H gas are, based upon all levels of methane processing, above the limits. A compliant mixture is not possible in this case, or needs to be tested on an

Table 13. Gas properties with maximum compliant LPG additions

in Vol.-% in Vol.-% in mol .-% in mol .-% in kWh/m³ in kWh/m³

Calorific

3,497 0,141 11,044 13,733 0,647

5,978 0,241 11,501 14,039 0,671

value Wobbe Index Relative

Density

Methane content in biogas

94,000

94,000

LPG as

3,7

6,5

individual basis.

admixture

Propane content in

admixture

**LPG-Mengen zur Erreichung des Zielbrennwertes**

Fig. 14. LPG quantities necessary to achieve the target calorific value

In order to achieve higher calorific values, alternative conditioning measures can be employed. For example, ready-made mixtures with customized propane / butane ratios can be used for conditioning. This can increase the calorific value, but technical and physical effects, such as condensation behaviour, methane number and k-number deviations need to be considered.
